The preparation of glass/plastic laminates for use in safety glazing applications presents unusually stringent manufacturing requirements in order to provide a product which exhibits acceptable optical quality.
Typically, glass/plastic laminates are produced in the following manner. A plastic composite having an adhesive side is placed onto a receiving glass sheet. A similarly shaped sheet of glass or rigid coverplate is placed over the surface of the composite. A vacuum is applied to the assembly or the assembly is pre-pressed to exclude air from between the layers. The assembly is then placed in an autoclave where lamination is completed by application of heat and pressure. In this operation the composite bonds to the glass, and the outboard surface of the composite is molded to replicate the surface of the coverplate.
Unfortunately, the resulting glass/plastic laminates do not always possess the desired optical qualities. During the bonding step any contaminating material between the coverplate and the surface of the plastic composite remains as a contaminant on the surface of the composite and may become embedded in the surface of the composite. After cooling, depressurization and coverplate removal, the contaminant leaves permanent, objectionable optical defects in the laminated structure. Damage done by very small particles can be observed by the naked eye. The visibility threshold for particles is typically 10 to 25 micrometers in diameter. However, a particle 3 to 5 micrometers in diameter can cause a visible defect in a glass/plastic laminate. Removal of the particles from the surface of the laminate after formation of the laminate does not cure the defect since the visual imprints made by the particles, i.e. depressions, remain on the plastic surface.
Efforts to solve this problem by modifying the surface of the coverplate have not been entirely successful. Obtaining optimum optical quality has required labor intensive cleaning procedures usually involving an expensive clean room atmosphere.
It is accordingly an object of the present invention to provide a process for producing laminates of improved optical quality. A further object is to provide glass/plastic laminates in which optical defects resulting from contaminants which may become embedded in the surface of the laminate during autoclaving or in other pressure/temperature processes are substantially eliminated without resorting to expensive clean room facilities and labor-intensive cleaning procedures.